The present invention relates to a multiple access method, to a transmitter and a receiver adapted to perform such a multiple access method.
Such a multiple access method, as well as a transmitter and a receiver adapted to perform such a method are already known in the art, e.g. from the article xe2x80x9cA New Look at Digital Orthogonal Transmultiplexers for CDMA Communicationsxe2x80x9d, by A. N. Akansu, M. V. Tazebay and R. A. Haddad, IEEE Trans. On Signal Processing, Vol 45, Nr1, January 1997. In FIG. 1b of that article, a code division multiple access, hereafter abbreviated with CDMA, transmitters and receiver are shown. Within a CDMA communications system each user network terminal is assigned a unique code to enable the central station which is coupled to it in a point to multipoint way, to discriminate the data from each of these network terminals on the basis of this unique code. Before upstream transmission of data by a network terminal, this code is multiplied with the oversampled data symbols by means of a sign change, which operation can be considered as equivalent to a discrete time filtering of this oversampled symbol with a filter having this code as its impulse response. In the receiver depicted in the prior art, a plurality of receive filters are present, whereby each of them is linked with a respective user network terminal, after which the signal is undersampled again.
Remark that throughout this whole document oversampling is to be considered as an operation on a discrete waveform, inserting a predetermined number of zeroes between two successive values of this discrete waveform.
The CDMA method has proven to be an efficient multiple access flat spectrum technique. However it looses efficiency when the users are not synchronous. Moreover when the communication channels are dispersive the ability to discriminate between several users vanishes.
An object of the present invention is to therefore provide a multiple access method, as well as a transmitter and receiver of the above known type but wherein the aforementioned problems of loss of orthogonality in the presence of dispersive channels or non-synchronous users, are solved, while still keeping the advantage of the flat spectrum.
According to the invention, this object is achieved by a multiple access method for use in a communications network wherein a central station is coupled to a plurality of network terminals in a point to multipoint way via the cascade connection of a common link and respective individual network terminal links, the method including a step of allocating a respective unique variable to each respective network terminal of the plurality, the method including a first filtering operation on at least one oversampled symbol which is derived from upstream information to be transmitted by a terminal of the plurality to the central station, the method including a next step of filtering a received oversampled symbol stream at the central station for enabling the central station to discriminate therefrom the at least one oversampled symbol that was earlier transmitted by the network terminal to the central station (CS), wherein the first filtering operation comprises an all-pass filtering operation of said at least one oversampled symbol, the all-pass filtering operation having a z-transform transfer function ({overscore (c)}i(z)) such that the zeroes thereof correspond to the respective unique variables assigned to all other network terminals, except the unique variable assigned to the network terminal, the next step of filtering comprises performing the z-transform on the received oversampled symbol stream, and of evaluating said z-transform at the unique variable (xcfx811;xcfx81i) assigned to network terminal.
The object of the invention is further achieved by a transmitter for use in a network terminal of the type described above, the transmitter including a first filter adapted to perform a first filtering operation on at least one oversampled symbol which is derived from upstream information to be transmitted by said network terminal, wherein the first filter comprises an all-pass filter, of which the zeroes of the z-transform transfer function comprise all of said respective unique variables, except the unique variable (xcfx811;xcfx81i) assigned to the network terminal.
The object of the invention is still further achieved by a receiver for use in a central station, the receiver includes second filter means (Z) adapted to perform a filter operation on a received oversampled symbol stream at said central station for enabling the central station to discriminate which part of the received oversampled symbol stream was previously transmitted by a network terminal of the plurality, wherein the second filter means (Z) comprises a z-transform means, adapted to perform the z-transform on the received oversampled symbol stream, and to evaluate the z-transform in the respective unique variable assigned to the network terminal.
In this way, as will be proven more into depth in the descriptive part of this document, by working now in the z-domain using all-pass filters for transmission, and by directly performing a z-transform in the receiver, whereby the zeros of these filters correspond to the discriminative variables of the user terminals, it is possible in the receiver to recover the original data stream transmitted by a particular user while nulling the contributions of all the others even in the presence of non-synchronous users and dispersive channels Moreover due to the all pass filter characteristics of the transmitter, a flat spectrum is preserved.
Another characteristic feature of the invention is that at least one respective unique variable assigned to at least one network terminal of the plurality comprises a plurality of unique variables assigned to the at least one network terminal, the zero factor (z-xcfx811) of the z-transform of the first filtering operation performed in all other network terminals and which has as zero the at least one respective unique variable of the at least one network terminal, thereby comprising a plurality of zero factors, with the plurality of unique variables as zeroes, whereby at the central station the z-transform is evaluated at each unique variable of the plurality assigned to at least one network terminal.
A still further feature of the invention is that the z-transform means in the receiver is further adapted to evaluate the z-transform at each unique variable of the plurality assigned to the network terminal (T1).
By this the robustness with respect to the noise is even increased. Indeed, by the assignment of a plurality of zeroess in stead of only one, averaging over several received signals transmitted by the same user is possible, thereby increasing the signal to noise ratio.
The present invention also relates to a network terminal and a central station which include a transmitter and a receiver of the type described above, and to a communications network wherein the aforementioned method is applied.
An additional characteristic feature of the present invention is that the method may include an additional step of measuring a channel parameter of the communication channel between the at least one network terminal and the central station, wherein the plurality of unique variables assigned to the at least one network terminal are thereby derived from the channel parameter. The communications network according to the invention may include a device adapted to measure this channel parameter. This allows to compensate for the frequency or phase shifts in the communications channel, for instance due to the Doppler effect, as this parameter can be measured and will be used for determining the plurality of zeroes assigned to at least one terminal. By communicating the thus determined zeroes to the plurality of terminals and to the receiver, and by performing these steps at regular instances, a dynamic assignment of zeros, allowing for a dynamic multiple access mechanism, is even obtainable.